A variety of diagnostic devices have been developed for the detection of an analyte of interest in a sample. In those devices in which sample collection and testing functions are non-linked, the transfer of collected sample to testing apparatus introduces a potential source of error. In those devices in which sample collection and testing functions are linked, the devices are dedicated in their entirety to the detection of a particular analyte and are not easily adaptable to a wide range of analyte detection.
The issue of self-diagnosis of serious disease such as cancer or AIDS has been considered at length. There is general consensus that self-diagnosis of such disease states is not preferred. Rather, it is generally accepted that a positive diagnosis for such a disease state should be communicated by a doctor, together with information relating to the availability of counseling services.
With respect to mammalian systems (e.g. humans), samples amenable to analysis using the testing device of the present invention include biological fluids (e.g. blood, urine, semen, saliva, etc.) or excrements. Such biological fluids can carry a variety of analytes, the presence of which can be diagnostic for a particular disease state. An important example of a disease state which is characterised by the presence of a disease-specific analyte in a biological fluid is Acquired Immunodeficiency Syndrome (AIDS). Using the device and method of the present invention, the presence of antibodies specific for the Human Immunodeficiency Virus (HIV), the causative agent of AIDS, in a blood sample are detectable.
The application of the subject invention to the detection of disease states in humans is of primary importance. However, in addition to use in the context of the diagnosis of serious disease states, the present invention is also useful in a variety of other contexts. Applications in connection with the analysis of microbes, plants, animals, food and water are all anticipated.
For example, ground water samples can be analysed for the presence of contaminants such as atrazine. Food, such as ground beef, can be analysed for the presence of contamination by bacteria such as E. coli. In the plant kingdom, the present invention can be applied to the analysis of, for example, pollen, spores and plant vascular fluids. Generally speaking, the only requirement for detection using the device and method of the present invention is that the analyte of interest should be soluble or suspendible in an aqueous solution.
The device and method of the present invention are particularly useful for the detection of occult gastrointestinal bleeding. The detection of occult gastrointestinal bleeding is a common method for screening for colo-rectal cancer. Commonly referred to as the faecal occult blood (FOB) test, a variety of formats are known in the art (see e.g., U.S. Pat. Nos. 3,996,006; 4,225,557; 4,789,629; 5,064,766; 5,100,619; 5,106,582; 5,171,528; 5,171,529; and 5,182,191).
The majority of test formats are based on the chemical detection of heme groups present in stool as a breakdown product of blood. In such tests, the pseudoperoxidase nature of the heme group is used to catalyse a colorimetric reaction between an indicator dye and peroxide. The oxygen sensitive dye can be gum guaiac, orthodianisidine, tetramethylbenzidine, or the like with guaiac being preferred.
While guaiac-based FOB tests are inexpensive and simple to use, there are disadvantages associated with their use. For example, guaiac-based tests indicate only the presence of peroxidase and pseudoperoxidase compounds, such as heme, that are present in a sample. Consequently, these tests are not specific for human blood, and are therefore subject to false-positive results if the patient's stool is contaminated with cross-reacting compounds. Such cross reacting compounds include, for example, non-human blood breakdown products from under-cooked meat, certain vegetable products, and some drugs. According to currently accepted medical practice, a patient demonstrating a positive result should then undergo a flexible sigmoidoscopy or colonoscopy to identify the source of the bleeding in the colon or rectum. These procedures can be invasive, medically complicated, and expensive. To minimize false positive reactions and the unnecessary follow-up procedures, guaiac based FOB tests require a restrictive diet for up to three days prior to testing.
Recent reports in the literature (Allison, et al. N. Eng. J. Med. 344: 155-159 (1996); and Favennec et al., Annales de Biologie Clinique 50: 311-313 (1992)) have suggested that screening by guaiac and confirmation of positive results by an immunological test, with absolute specificity for human blood, would increase the value of FOB test results. By this means, only those patients with confirmed gastrointestinal bleeding would be subjected to the expensive follow-up procedures, leading to significant savings in health care delivery cost and reduced inconvenience to the patient.
The present invention relates to a device which is useful inter alia for the detection of any aqueous soluble or suspendible analyte which is detectable, for example, on the basis of immunological and/or chemical properties. An example of an analyte detected by its immunological properties includes, but is not limited to, an immune interacting molecule such as an antigen, hapten, immunoglobulin or T-cell derived antigen binding molecule. An example of an analyte detected by chemical properties includes an enzyme, catalyst or ligand. Thus, with respect to FOB tests, the device of the present invention can be adapted to either guaiac-based testing, or immunological testing. The preferred format for immunological testing is immunochromatography. This format is described generally in U.S. Pat. Nos. 5,591,645 and 5,622,871, the disclosures of which are incorporated herein by reference.
Prior to discussing the invention in greater detail, a brief review of the immunochromatography process will be provided to establish certain principles. To detect an analyte of interest by immunochromatography, two binding reagents which bind specifically and non-competitively to the analyte of interest may be employed. A first specific binding reagent is labelled and is free to migrate. When introduced to a sample to be tested for the presence of the analyte of interest, the first specific binding reagent binds to the analyte of interest, if present. The second specific binding reagent is immobilized in a detection zone on a liquid-conductive solid phase material, the detection zone being remote and downstream from the location of initial contact between the first binding reagent and the analyte of interest. A solvent front carrying the mobile first specific binding reagent complexed with analyte of interest (if present) migrates along the liquid-conductive solid phase material through the detection zone. If analyte is present in the sample, the immobilised second specific binding reagent binds the analyte thereby forming an immobilised sandwich complex comprising the first specific binding reagent (which is labelled), the analyte of interest, and the second specific binding reagent (which is immobilised). Detection of the label immobilised in the detection zone is indicative of the presence of analyte of interest in the sample. In most embodiments, the first and second specific binding reagents are either polyclonal or monoclonal antibodies.